1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to systems and techniques for providing conference calling in digital telephone systems.
2. Description of the Related Art
Generally, communications systems transmit voice, video and/or data from one point to another. In certain applications, it is desirable to communicate between multiple points. In a voice context, this is achieved by `conference calling`. Conference calling allows each of a plurality of spatially separated participants to communicate contemporaneously with each other as though present in a shared environment.
In a conventional (land line) telephone system, voice quality for conference calls is maintained through the use of switches and amplifiers which sum and distribute the speech of those speaking to the participants in the conference call.
However, in digital wireless communication systems such as cellular telephone systems, conference calling is somewhat more problematic. This is due to the fact that speech in these systems is generally compressed at some point in transmission. Conference calling in this context conventionally requires decompression of compressed speech so that the signals may be summed as analog signals prior to any re-transmission to conference participants. This is due to the cost and complexity associated with digital signal summing schemes. Unfortunately, the compression, decompression, recompression and re-decompression of speech significantly degrades the quality thereof at the receiver. An excellent illustration of this problem may be found in a CDMA conference bridging system.
CDMA (Code Division Multiple Access) modulation is one of several techniques for facilitating communications in which a large number of system users are present. CDMA offers several advantages over other techniques known in the art such as TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), and AM (Amplitude Modulation) schemes such as ACSSB (Amplitude Companded Single Sideband). The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", the teachings of which are incorporated herein by reference. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", the teachings of which are also incorporated herein by reference.
CDMA systems often employ a variable rate vocoder to encode data so that the data rate can be varied from one data frame to another. An exemplary embodiment of a variable rate vocoder is described in U.S. Pat. No. 5,414,796, entitled "VARIABLE RATE VOCODER", the teachings of which are incorporated herein by reference. The use of a variable rate communications channel reduces mutual interference by eliminating unnecessary transmissions when there is no useful speech to be transmitted. Algorithms are utilized within the vocoder for generating a varying number of information bits in each frame in accordance with variations in speech activity. For example, a vocoder with a rate set comprising four rates may provide 20 millisecond data frames containing 20, 40, 80, or 160 bits, depending on the activity of the speaker. It is desired to transmit each data frame in a fixed amount of time by varying the transmission rate of communication. Additional details on the formatting of the vocoder data into data frames are described in U.S. Pat. No. 5,511,073, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", the teachings of which are incorporated herein by reference.
In a conventional CDMA conference bridging arrangement, the speech signals received from each of the participants are devocoded, summed then revocoded and re-transmitted to the participants. Devocoding involves detecting the rate of the received vocoded signal and decoding it accordingly. One technique for determining the rate of a received frame of vocoded data is disclosed and claimed in copending U.S. patent application Ser. No. 08/233,570, filed Apr. 26, 1994, U.S. Pat. No. 5,566,206, and entitled "METHOD AND APPARATUS FOR DETERMINING DATA RATE OF TRANSMITTED VARIABLE RATE DATA IN A COMMUNICATIONS RECEIVER", and U.S. patent application Ser. No. 08/126,477, entitled "MULTIRATE SERIAL VITERBI DECODER FOR CODE DIVISION MULTIPLE ACCESS SYSTEM APPLICATIONS", filed Sep. 24, 1993, abandoned, the teachings of both of which are incorporated herein by reference. According to these techniques, each received data frame is decoded at each of the possible rates. Error metrics, which describe the quality of the decoded symbols for each frame decoded at each rate, are provided to a processor. The error metrics may include Cyclic Redundancy Check (CRC) results, Yamamoto Quality Metrics, and Symbol Error Rates all of which are well-known in the art. A processor analyzes the error metrics and determines the most probable rate at which the incoming symbols were transmitted. This rate is used to decode the received signal.
In a conference bridging arrangement, the speech and/or background noise from each conference participant is vocoded and transmitted to a base station by a transmitter at each subscriber station. At the base station, the vocoded signals are received, devocoded, summed, revocoded and re-transmitted to the conference participants. At each subscriber location, the revocoded signals are re-devocoded. As mentioned above, this vocoding, devocoding, revocoding and re-devocoding severely degrades the quality of the received signals.
Hence there is a need in the art for a system and/or technique for maintaining voice quality in wireless digital conference calling systems.